Vehicles with short tractor units are no more dangerous than others

VTI has contributed with simulations of different tractor and semi-trailer combinations. From December 2022, new rules apply to tractor units with semi-trailers, so-called EU trailers, according to a government decision from September. The decision has been made after the Swedish Transport Agency, with the help of VTI, investigated appropriate safety-enhancing measures for EU trailers.

The new regulation entered into force on 1 December 2022. The change in the regulations aims to increase road safety and accessibility for this type of vehicle in slippery road conditions by setting requirements for the distribution between wheel loads on the drive axle and other axles of the vehicle.

There has been a concern that these vehicles lead to road accidents, especially in winter road conditions. The VTI researchers who have investigated different vehicle combinations through simulation with computer models believe this may be unfounded. The report is funded by the Swedish Transport Agency and is part of a larger investigation.

There are EU regulations that limit the overall length of vehicle combinations, which has led to tractor units with short axle lengths. Heavy vehicles from the continent are accident-prone, and there has been a suspicion that this is due to them being designed differently compared to those that are common in Nordic countries.

According to Sogol Kharrazi, senior researcher at VTI and one of the authors, “the concern was about the negative consequences for road safety, that short tractors would have a poorer ability to handle uphill climbs and that the tractor unit's short wheelbase would cause jackknifing, for example when braking and turning.”

Jackknifing is the folding of an articulated vehicle so that it resembles the acute angle of a folding pocketknife so that the trailer's rear end swings to the side.

The researchers have compared four different combinations where the shortest length of the vehicle is 16.5 m and the longest is 25.25 m. These have been short, as well as long, tractors with semi-trailers, trucks with a dolly and a semi-trailer, also called Nordic combination, as well as tractors with link trailer and a semi-trailer.

The simulations have involved some important situations and manoeuvres where a comparison makes sense, such as braking in a curve and starting on a hill, especially in winter conditions. The findings demonstrate that neither the coupling distance nor the wheelbase are crucial factors. There are other things that affect performance more, especially a working ABS system or good lubrication of the fifth wheel.

“More efficient braking provides better grip in dangerous traffic situations because friction determines how the trailer moves. Also important is the condition of the fifth wheel, i.e. the fifth-wheel coupling that connects the tractor unit with the semi-trailer,” says Sogol Kharrazi.

The researchers provide four recommendations. Requirements for a functioning ABS system on all axles. Caution with engine braking in slippery winter conditions. Taking measures to cope with uphill climbs in slippery conditions by temporarily exceeding the load limit. The fifth wheel needs to be lubricated, and the lubricant must be able to withstand winter conditions.

According to Mattias Hjort, research leader at VTI, “the government's decision means that it has not followed any of the recommendations given by VTI, but has only formally required a certain percentage of weight to be on the drive axle, a measure that primarily aims to curb the improper loading of vehicles.”

The report is written in English. In addition to Sogol Kharrazi, the report is authored by senior researchers Fredrik Bruzelius, Mattias Hjort and Bruno Augusto, research engineer.

Text: Gunilla Rech

Translated by: CBG Konsult & Information AB

The influence of tractor lengths on traffic safety and efficiency: a simulation study External link.

Don´t miss out on our news – subscribe now!

Stay informed with the latest research and news from VTI. Sign up for newsletters, sent by e-mail four times a year.